How Tos / Tech

Please Release Me: How to Set Up a Hydraulic Clutch

 

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This is the slave cylinder tucked up against the bellhousing on a fourth-gen Camaro. GM used a pull-style slave cylinder clutch system on the Tremec T56 from 1993 to 1997.

Here is the Camaro’s clutch master cylinder buried under the cowl behind the brake booster. In this case, the reservoir is mounted remotely due to the cramped quarters.

This is a Wilwood clutch slave cylinder. The slave cylinder is manufactured from billet aluminum and has a stainless steel pushrod with a longer stroke (1.38 inches) than most cylinders. That guarantees full clutch release. Maximum input pressure is 1,200 psi.

Wilwood designed its slave cylinder for use with a 7/8-inch bore master cylinder. Summit Racing offers a nice selection of Wilwood master cylinders; this kit is common for use with a hydraulic clutch.

McLeod’s release bearing is engineered to float on the transmission input shaft, and installs just like a mechanical clutch release bearing. One hose goes to the clutch master cylinder; the other is a bleeder hose.

Tilton’s bolt-in hydraulic release bearings physically bolt to the inside of the bellhousing or to the transmission input shaft. When mounted directly to the gearbox, the stock input shaft retainer must be swapped for one of Tilton’s retainers. If you have a Chevy transmission, use one of Tilton’s dedicated bellhousings.

Tilton’s bellhousings accept the company’s four-leg hydraulic release bearing. As you can see it bolts directly to the bellhousing. Note the -3 AN ports (red caps). One goes to the master cylinder, the other is a bleeder.

Here’s the clutch pedal we mentioned our example for determining pedal ratio. Measurement #1 is the distance from the very base of the pedal to the centerline of the pivot point. The measurement came out to 13.25 inches.

Measurement #2 in our pedal ratio calculation is the distance from the centerline of the pushrod hole to the centerline of the pivot point. In our case, there are two master cylinder pushrod holes in the pedal that measure 3 inches and 3.625 inches respectively from hole-to-pivot-point centerline. To determine the pedal ratio, divide measurement # 1 by #2. The upper hole provides 4.41 pedal ratio, the lower hole a 3.63 ratio. Neither is an optimal ratio. Drilling a new pushrod hole 2.2 inches from the pivot point provides a much better 6:1 pedal ratio.

There are two ways you can release a clutch on a car or truck—with mechanical linkage or via hydraulics. In this story, we’ll explore the hydraulic clutch.

Slave Cylinders vs. Hydraulic Bearings

There are two basic types of hydraulic clutches: slave cylinder and release bearing. In a slave cylinder system, a hydraulic cylinder is installed on the bellhousing and connected to a mechanical clutch fork. A clutch master cylinder feeds the slave cylinder. When you press on the clutch pedal, the pressure forces fluid from the master cylinder to the slave; the slave cylinder moves the clutch fork, which in turn activates the clutch release bearing.

Detroit has used various configurations of this system over the years. In some systems, the slave cylinder pulls the clutch fork; in others the slave cylinder pushes against the fork. The photo captions above will show you how a typical later model Chevy slave cylinder system is set up.

Several aftermarket manufacturers offer quality slave and master cylinders. Wilwood’s “pull type” slave setup is a good example. The slave cylinder is manufactured from billet aluminum and has a stainless steel pushrod with a longer stroke (1.38 inches) than most cylinders. That guarantees full clutch release. Wilwood designed the slave cylinder for use with a 7/8-inch bore master cylinder. If you have packaging issues under the hood, Wilwood offers their Compact Series slave cylinder. It has an ultra-short 3.37-inch body but provides 1.12 inches of stroke.

Slave cylinder systems are simple, but the hydraulic release bearing is simpler still. There is no slave cylinder or clutch fork. Instead, the clutch master cylinder applies hydraulic pressure directly to the bearing that engages the pressure plate fingers. A hose running directly from the clutch master cylinder to the master cylinder replaces virtually all of the mechanical parts.

There are several different designs of hydraulic release bearings available. One is the floating bearing offered by McLeod and others. This type of bearing works much like a regular release bearing, except it is engaged via hydraulics instead of mechanics. The other type is the fixed release bearing offered by Tilton. The bearing is either physically bolted to the inside of the bellhousing (you have to use a dedicated Tilton bellhousing for this) or it can be bolted to the transmission. When mounted directly to the gearbox, the stock input shaft retainer must be swapped for one of Tilton’s retainers. Tilton offers bearing/retainer packages for everything from vintage Muncies to modern Tremecs. They also offer a generic input shaft retainer that you can machine for not-so-common applications.

High performance hydraulic release bearings are “constant contact” types that  provide quick clutch response. You will need to set a small operating clearance between the bearing and the pressure plate. Once you have the bearing properly adjusted for clearance, it will self-adjust for wear. That’s because there is no return spring required to pull the bearing’s piston back to the bottomed position. The piston within the assembly functions much like the piston in a brake caliper. Clutch pedal feel remains the same as the bearing wears, too.

Installation of a hydraulic release bearing is straightforward: Most bearings have a straight -3 AN union fitting installed for the master cylinder plumbing as well as a second -3 AN fitting for a bleeder hose. Tilton recommends a 7/8-inch bore master cylinder for a street clutch, while McLeod usually recommends a 3/4-inch bore master cylinder.

Determining Pedal Ratio

The key to a hydraulic clutch setup is getting the pedal ratio correct. In fact, it’s critical, and an inch or two can make all the difference in how the clutch performs. Just like a brake pedal, the clutch pedal acts as a lever to increase the force applied by the driver to the master cylinder. That force tells the master cylinder to send fluid to the release bearing or clutch slave.

If you examine a clutch pedal, you’ll see the pivot point (where the pedal moves) and the mounting point/hole for the master cylinder pushrod are quite often different. By varying the length of the pedal and/or the distance between the pushrod mount and the pedal pivot, you can change how much force (via your left leg) is required to energize the master cylinder. This is the mechanical advantage, or pedal ratio. What you need to do is to figure out a ratio that provides enough force to energize the master cylinder without requiring too much effort on your leg’s part.

Let’s start at the beginning. The typical adult male leg can exert roughly 300 pounds of force. If you’ve ever tried to single-leg press 300 pounds at a gym, you know that’s a bunch. When you’re figuring out the right pedal ratio for the clutch, shoot for a force number that is 1/3 or less than that 300 pound figure. That will make working the clutch much more comfortable.

Next, you have to translate that 100 or so pounds of leg force into approximately 600 psi. You can do this by changing the overall length of the pedal, but it’s usually easier (and far more practical) to shorten the distance between the pivot point and the master cylinder pushrod mount location by drilling a hole for a new pushrod location.

If you go overboard in the pedal ratio department, the clutch can become over-sensitive or touchy. So how do you get it right?  First, you need some measurements:

  1. Height of the pedal from the very bottom through the centerline of the pivot point
  2. Center-to-center measurement through the pivot point and the clutch pushrod hole in the pedal lever

Using an aftermarket reproduction clutch pedal as the example, dimension #1 is 13.25 inches. There are two different stock pushrod holes available (measurement #2): three inches and 3.625 inches. To determine pedal ratio, divide figure #1 by #2. Here’s how the ratios look for our example:

13.25 / 3 = 4.41 ratio

13.25 / 3.625 = 3.63 ratio

The recommended pedal ratio with either a 7/8- or 3/4-inch bore clutch master cylinder is around 6:1, so we would need to drill a new pushrod hole approximately 2.2 inches from the pivot point to get the optimum pedal ratio (13.25 / 2.2 = 6.072).

Take a look at the photos and captions above to see how simple a hydraulic clutch system really is. In a few weeks, we’ll look at some mechanical linkage options.

 

 

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